The present invention relates to a dryer section for a paper machine which comprises at least dryer groups provided with single-wire draw. Each of the single-wire draw groups comprises a number of drying cylinders and a number of reversing cylinders or rolls arranged in the gaps between the drying cylinders. In each of the dryer groups, a web runs under constant contact with the wire over the drying cylinders and reversing cylinders or rolls so that the web enters into direct contact with the drying cylinders and the wire enters into direct contact with the reversing cylinders or rolls.
Currently the highest web speeds in paper machines are of an order of about 25 meters per second, but before long the speed range of from about 25 m/s to about 40 m/s is also likely to be realized. With the current highest speeds and with the future still higher speeds, in particular the dryer section has become and will be a bottle-neck for the runnability of a paper machine. With a view toward obtaining an adequate drying efficiency, the dryer section has often become long, which increases the costs of the dryer section and of the machine hall.
In the prior art, in multi-cylinder dryers of paper machines, twin-wire draw and/or single-wire draw is/are employed. In the former case, the groups of drying cylinders comprise two wires which press the web, one from above and the other one from below, against the heated cylinder faces of the drying cylinders. Between the rows of cylinders, which are usually horizontal rows, the web has free and unsupported draws which are susceptible of fluttering and which may result in web breaks. In the single-wire draw, each group of drying cylinders comprises a single drying wire on whose support the web runs through the whole group so that, on the drying cylinders, the drying wire presses the web against the heated cylinder faces, and on the reversing cylinders between the drying cylinders the web remains at the side of the outside curve, i.e., the drying wire is between the web and the outer surface of the reversing cylinders. Thus, in single-wire draw, the drying cylinders are placed outside the wire loop, and the reversing cylinders inside the loop. In the prior art normal groups with single-wire draw, the heated drying cylinders are placed in an upper row, and the reversing cylinders are placed in a lower row, the rows being generally horizontal and parallel to one another. So-called inverted groups with single-wire draw are also known, in which the heated drying cylinders are placed in the lower row and the reversing suction cylinders or rolls in the upper row, the substantial objective of an inverted group being to dry the web from the side opposite in relation to a normal group with single-wire draw.
In the area of the dryer section of a paper machine, various problems have occurred, of which in particular the large length of the dryer section should be mentioned. With respect to the prior art related to this, reference is made to U.S. Pat. No. 5,177,880, in which a dryer section of a paper machine is described which has been divided into a number of dryer groups, each of which groups comprises a number of drying cylinders, a number of reversing cylinders in the gaps between the adjacent cylinders, and a web support belt which runs around the cylinders in the dryer group. In each dryer group, the web runs under constant contact with the support belt over the drying cylinders and the reversing rolls so that the web enters into direct contact with the cylinders and that the support belt enters into direct contact with the reversing rolls. The cylinders are arranged in a number of rows, which rows are inclined in relation to the vertical direction alternatively rearward or forwards, thus defining V-shaped double rows. The cylinder placed at the end of each row and the cylinder placed at the beginning of the next row form a pair of cylinders, which cylinders are arranged horizontally side by side. The drying cylinders follow each other as a zig-zag line. Each inclined row comprises about three cylinders.
In the prior art, constructions are also known in which the cylinders are placed in vertical rows. One such construction is described in U.S. Pat. No. 4,972,608.